Why doesn't GCC optimize a*a*a*a*a*a to (a*a*a)*(a*a*a)?

Question

I am doing some numerical optimization on a scientific application. One thing I noticed is that GCC will optimize the call pow(a,2) by compiling it into a

Answer 1

There are already a few good answers to this question, but for the sake of completeness I wanted to point out that the applicable section of the C standard is 5.1.2.2.3/15 (which is the same as section 1.9/9 in the C++11 standard). This section states that operators can only be regrouped if they are really associative or commutative.